The recent spread of handheld terminal devices such as laptop computers, cellular phones, and PDAs has been remarkable. As the secondary battery power source in these handheld terminal devices, a non-aqueous electrolyte secondary battery, such as a lithium ion secondary battery (hereinafter sometimes referred to simply as a “battery”) is often used. Handheld terminal devices need to have a comfortable portability, and thus such devices are rapidly becoming more compact, thinner, and lightweight with better performance. Consequently, handheld terminal devices are now being used in a wide variety of situations. Further, similar to handheld terminal devices, other batteries now used as a power source due to the expansion of how devices are used also need to be more compact, thinner, and lightweight with better performance. Therefore, there is a need for batteries to have a greater capacity.
Therefore, attempts to improve the performance of the various parts and materials in lithium secondary batteries are actively being pursued. Among such efforts, battery capacity improvement resulting from the negative electrode active material is becoming more important.
As the negative electrode active material, currently, carbon active materials such as graphite are mainly employed, whereby a discharge capacity of about 350 to 360 mAh/g, which is close to the theoretical capacity for graphite of 372 mAh/g, is realized. However, it is impossible to exceed the theoretical capacity of graphite. In view of this limit due to the theoretical capacity of graphite, research has been carried out into metals capable of storing Li. As a result, it has been found out that a high theoretical capacity can be achieved with an alloy active material of silicon, tin, zinc, aluminum, gallium, copper, silver, germanium, indium and the like. Among these, silicon has a far greater theoretical capacity of 4,200 mAh/g, although the adhesion strength of the electrode is disadvantageously lowered due to the expansion and contraction that occurs during charging and discharging. Consequently, the negative electrode materials degrade, which shortens the cycle life of the battery. Thus, in order to increase the discharge capacity and improve the cycle property as well, alloy active materials have been proposed which are a mixture of silicon and graphite powder, or are formed by adhering a silicon powder on the surface of a carbon powder or a graphite powder by mixing a carbon powder or graphite powder with a silicon powder, and then coating pitch on the resultant surface.
For example, Patent Document 1 has proposed use of a negative electrode obtained using a slurry obtained by dispersing a silicon compound and a carbon material in a binder such as polyester. In Patent Document 1, the binder offsets the volume changes of the silicon compound negative electrode during doping and undoping of lithium, which suppresses the volume changes of the whole active material layer, thereby suppressing cycle deterioration.
Further, Patent Document 2 has proposed a negative electrode material for a lithium secondary battery that includes silicon particles and a plurality of types of carbonaceous material, and that has voids. As the plurality of types of carbonaceous material, Patent Document 2 discloses a carbonaceous material A having a comparatively high level of electron conductivity selected from graphite, carbon black, carbon nanotubes, carbon fibers and the like, and a carbonaceous material B, which is an amorphous carbon selected from a pitch-based material, a tar-based material, a resin-based material and the like. In Patent Document 2, the carbonaceous material B or a precursor of the carbonaceous material B, silicon particles, and the carbonaceous material A are mixed with a twin-screw heating kneader or the like, and then thermally treating the resultant mixture to obtain composite particles. Using these particles as the negative electrode material, the expansion and contraction of the silicon that occurs during charging and discharging is alleviated, whereby the aforementioned cycle property problem is diminished.